“TinyUPS is a simple 5V/2.5A uninterruptible power supply with a li-ion battery as a buffer, a load sharing power path management system and an ATtiny13 for monitoring power supply and battery charge level as well as for communication with the connected device.
If external power is connected to the tinyUPS the input voltage or vcc of the ATtiny13 is delivered by this source, otherwise by the battery. The ATtiny13 monitors the input voltage and tells the connected device to shutdown by pulling the SHUTDOWN-line low when the input voltage falls below a certain threshold (SHUTDOWNLEVEL). This happens when the external power source is diconnected or disabled and the battery level falls below this threshold. After waiting a certain time (SHUTDOWNTIMER) to allow the connected device to safely shut down, the ATtiny13 deactivates the boost converter and turns off the power to the connected device. If the input voltage rises again above a certain threshold (POWERONLEVEL) it activates the boost converter and turns on the power to the connected device. This happens when the external power source is available again. When power is turned on a BOOTUPTIMER starts to count. If a shutdown is initiated before the boot up is completed, the left-over time is added to the SHUTDOWNTIMER in order to allow the connected device to completely boot up and shut down. A shutdown can also be initiated by pressing and holding the button or by setting the REQUEST-line to high (>0.7V) for 2 seconds. After such shutdowns the power will not be turned on again automatically. The power to the connected device can be turned on manually by pressing the button or setting the REQUEST-line to high if the battery level is above a certain threshold (USERPOWERLEVEL) or the external power source is connected.
The SHUTDOWN pin of the tinyUPS is an open collector output. The connected device must have an internal or external pullup resistor on the SHUTDOWN line! This is necessary because of the different voltage levels.”